1. Field of the Invention
This application relates generally to printers and more particularly to a method for reducing pixel density in pixel-array printers.
2. Description of the Related Art
Printing devices used to provide a hard copy version of images, text, or graphics (collectively images) produced in a computer system are almost exclusively binary in nature. Such devices include ink jet printers, dot matrix printers, laser printers, and thermal printers. After appropriate processing, the image to be printed is typically represented by a large two-dimensional array of uniformly spaced and sized pixels, in which each individual pixel is either xe2x80x9conxe2x80x9d or xe2x80x9coffxe2x80x9d. The printing device produces a mark on the output medium corresponding to each pixel that is marked xe2x80x9con.xe2x80x9d The intrinsic binary nature of the printed image becomes less obvious and the perceived image quality improves as the number of pixels per unit area increases.
Modem printers often include a provision for printing an image with reduced pixel density. The process of reducing the pixel density is alternatively referred to as dot depletion, particularly in the context of ink jet printing. The reduced pixel density corresponds to a draft mode or economy mode which advantageously uses less ink and also may provide faster throughput.
Depletion may also be employed to overcome problems associated with printing dense areas of an image. In areas of high density, the ink may not dry quickly enough, leading to smearing when two pages touch or when the user picks up the page. Also, in areas of high density, ink may be wasted as drops overlap each other. An equally dense image may be achieved with less ink and fewer drying problems. Finally, ink in high density areas may wick to lower density areas, degrading the image quality.
Furthermore, dot depletion techniques can also provide an enhanced mode of printing in which higher resolution data can be printed by a device designed for lower resolution than the data to be printed. These techniques are described, for example, in U.S. Pat. No. 5,574,832, which is commonly assigned with the present application and incorporated herein by reference.
Algorithms have been developed to determine which pixels to remove to achieve the benefits of depletion while maintaining print quality. These algorithms are typically executed on the array of pixels that is otherwise ready for printing. It would be desirable to provide a method of depletion that avoids having to execute additional time-intensive algorithms.
A method of reducing pixel density in pixel array printers uses information about a pixel when it is first available instead of having to recalculate that information at a later time.
The method determines whether a pixel is located at an edge of a feature or at the center of a feature. In one embodiment, pixels located at an edge are printed in full, that is, all xe2x80x9conxe2x80x9d pixels are left xe2x80x9conxe2x80x9d while depletion is applied to central, alternatively termed xe2x80x9carea-fillxe2x80x9d pixels. Alternatively, edge depletion could be performed in which edge pixels are depleted and area-fill pixels are printed in full.
Image data to be printed in pixel array printers is typically available in computer memory in a continuous color scale version (RGB data) suitable for display on computer monitors. To print on a binary printing device, this version has to be converted to a binary version of the image. The conversion methods are commonly referred to as halftoning.
According to embodiments of the present invention, the process of identifying edge pixels for the purpose of depletion is performed during the halftoning process, in which every pixel in a pixel data array is examined. Pixels are identified with one of four possible pixel states: a blank pixel, a left/right edge pixel, a top/bottom edge pixel, or an area-fill pixel. The pixel state information is stored in a modified pixel data area. A predetermined pattern of filled and unfilled smaller pixels is identified with each pixel state. In one embodiment, the predetermined pattern corresponds to all filled smaller pixels for the edge pixel state and a pattern of filled and unfilled smaller pixels for the area-fill pixel state. In this way, the portions of the pixel array corresponding to area fill pixels are depleted. Each element in the modified pixel data array is mapped onto the smaller pixels according to the predetermined pattern and a printer is controlled according to the resulting pattern.
According to other embodiments of the present invention, edge identification for the purpose of depletion is combined with algorithms, other than halftoning algorithms, such as compression or black fortification algorithms, that process every element of a pixel data array.